In the art of papermaking, the use of extended nip presses--also referred to as long nip presses or wide nip presses--in the press section of a papermaking machine has become very popular since the extended nip press has a dewatering capacity that is significantly larger than the dewatering capacity of a conventional roll nip press. The extended nip presses marketed today by various manufacturers are typically of the kind referred to as shoe presses
A typical shoe press unit comprises a generally concave shoe, a rotatable flexible tubular jacket running in a loop around the shoe, a stationary support beam for supporting the shoe, at least one (normally several) actuator(s) on the support beam for pressing the shoe against an interior surface of the jacket and a cylindrical counter roll. The shoe is often lubricated to avoid wear with the inner surface of the tubular jacket. The concave shoe and the cylindrical counter roll define between them an extended nip through which a fibrous web is passed.
In comparison with a conventional roll press, an important advantage of a shoe press is that a higher linear loading may be used than in a roll press. The amount of dewatering which takes place in a press nip depends to a large degree on the press impulse that can be calculated as I=L/S where I=the press impulse, L=the linear loading (force per unit length in the cross machine direction) of the press and S=the speed of the paper web through the nip. In theory, the press impulse does not depend on the length of the nip in the machine direction. However, the maximum linear loading that can be applied in the press nip is limited by the pressure to which the web can be subjected.
In a conventional roll press, the nip area is very small and even a relatively low linear loading may result in a pressure which is simply too high for the fibrous web that is passed through the nip. A shoe press with its long nip and large nip area can use a much higher linear loading and yet not reach such high levels of pressure in the nip that crushing of the fibrous web will occur. This is of particular importance for the papermaker who desires to obtain a high bulk product. Since the shoe press can use a high linear loading without subjecting the fibrous web to a high pressure, a high bulk product can be obtained. Therefore, the first commercial use of shoe presses was in machines for making high bulk products, such as board machines. Recently, shoe presses have also been employed for different paper grades.
However, a high dewatering capacity is not always the only desired property of a press unit. For example, it might also be desirable that the final product has a high strength in terms of Scott Bond. In order to obtain a paper with high strength in terms of Scott Bond, the fibrous web should be subjected to a high pressure as it is passed through the press unit. An easy way to obtain a high pressure in the nip is to use a conventional roll press unit. However, a roll nip is not compatible with the requirement for a high dewatering capacity as explained above. In addition, high pressure in the nip may result in a product having insufficient bulk.
An alternative solution would be to use a shoe press and apply a higher linear loading than usual. However, a shoe press is designed to result in a relatively low pressure and even with a high linear loading it can be difficult to obtain a very high level of pressure. Furthermore, if the pressure level in a shoe press nip is raised, frictional heat generated between the flexible jacket and press shoe may become a very serious problem, even if the shoe is lubricated. The frictional heat that is generated may cause deformation of the shoe and other problems associated with dissipating the generated heat, such as overheating of the flexible jacket. The frictional heating can become especially serious if the papermaking machine is run at high speeds such as 25 m/s or higher.
In order to solve the problem of frictional heat generated in a shoe press nip, it has been suggested in U.S. Pat. No. 4,643,802 (Schiel) to use a heat insulating layer between an upper and a lower part of the shoe. However, such a solution makes the shoe more complicated and expensive to manufacture. Furthermore, the use of a heat insulating layer only prevents heat from being conducted to other parts of the machinery, and does not decrease the amount of heat transferred to the tubular jacket.
The amount of frictional heat developed in an extended nip depends on a number of parameters such as nip pressure, machine speed and the lubrication system. For lubrication, a shoe press can be hydrodynamically lubricated or hydrostatically lubricated. In a hydrodynamically lubricated shoe, lubrication oil can be sprayed into an interface between the flexible jacket of the shoe press and a leading edge of the shoe itself as disclosed in e.g., U.S. Pat. No. 5,167,768 (Cronin et. al). In a hydrostatically lubricated shoe, lubrication oil is fed through a conduit in the shoe to a lubrication pocket on the face of the shoe. Such a lubrication system is disclosed in for example U.S. Pat. No. 5,262,011 (Ilmarinen). A hydrostatically lubricated shoe normally also has one or more regions or zones of hydrodynamic lubrication at least adjacent to the leading and/or trailing edges of the hydrostatic lubrication region. The Ilmarinen patent discloses leading and trailing land surfaces which are both concave and have a radius of curvature corresponding to that of the counter roll, thus creating a shoe that is both hydrodynamically and hydrostatically lubricated.
Frictional heat in a shoe press nip is generated in large part by the hydrodynamically lubricated zones. Therefore, the problem of frictional heat could theoretically be overcome by using a shoe that is completely hydrostatically lubricated. It has been suggested, see for example German Patent DE 35 03 819 to Sulzer Escher-Wyss GmbH, that hydrodynamic lubrication be entirely eliminated. The German '819 patent discloses a kind of press shoe which consists almost entirely of a hydrostatic pocket. In theory, such a shoe would generate only a very small amount of frictional heat and could be suitable for high nip pressures in combination with high machine speeds. However, the shoe disclosed in the German '819 patent has a sharp transition from the hydrostatic pocket to the end walls of the pocket. In the transition area, the flexible jacket will be subjected to stress and wear and possibly a considerable degree of heat generation.
The inventor has found that fibrous webs made from wood-containing stock are less pressure sensitive and can be subjected to a high pressure without significant loss of bulk. For such paper grades, for example supercalendered (SC) or light weight coated (LWC), it would be advantageous if the web could be pressed in a press unit with both a high degree of dewatering and a high nip pressure. The inventor has also recognized that this would be best achieved if the web could be passed through a shoe press unit which employs a pressure which is higher than a conventional shoe press (but still lower than a roll press). However, if a conventional shoe press unit employs high pressures of the magnitude required for desired dewatering and Scott Bond strength, excess frictional heat generated in the press nip will become a problem.
Therefore, there is a need for a shoe press that can employ a high nip pressure at high speeds for obtaining a desired level of dewatering and Scott Bond strength. However, such a shoe press should not be subject to overheating of the shoe or jacket and should not suffer from accelerated wear characteristics.